I am taking out a 50 gallon hot water heater and replacing it with a tankless. The first problem is taking out the old tank. It does not have a typical union at both the hot and cold water pipes. It looks like a fitting which can be rotated and both ends should rotate out. I have never used this type before and it is somewhat stuck. Should I take a torch to it to loosen it, or just cut it and either use a compression fitting or sweat solder a new fitting which can be connected to the hot water heater?

I would just cut the fitting out and couple to the hot and cold lines using either a compression or a sweat fitting. Your choice. Regards. Tom